Acetylene ring closures

The stereoselective synthesis of y-bisabolenes was made possible by the development of a new method for the carbosilylation and double alkylation of an acetylenic function coupled with ring closure, overall addition of three carbon substituents to two acetylenic carbons. 

The ring closure seems to involve intramolecular attack of the deprotonated methylene group of primary adduct at the second acetylene bond (75DIS). 

Dialkyltriazenes ortho to an acetylene substituent on an aryl ring as in 184, undergo ring closure in modified Richter-type cyclizations, forming substituted cinnolines 185 and isoindazoles 186 in fair to very good yields . 

A titan acycl open tadi en e generated from an acetylene having an ester group at a suitable position reacts intramolecularly with this functional group, as exemplified by Eq. 9.13. Here, both carbon—titanium bonds of the titanacycle participate in the reaction to effect ring-closure [33]. 

Pattenden and Teague have prepared tricyclic diol 684 which is epimeric to the naturally occurring A < -capnellene-8p,10a-diol (68S) Their strategy, which is summarized in Scheme LXXI, encompasses two critical cyclization steps. The first is the Lewis acid-catalyzed ring closure of enol acetate 686 and the second involves reductive closure of acetylenic ketone 687. It is of interest that the oxidation of 688 proved to be stereospecific. 

Base-induced isomerization of propargyl amide 29a gives chiral ynamide 30a, which is subjected to ring-closure metathesis to afford cyclic enamide 31a. Diels-Alder reaction of 31a with dimethyl acetylene dicarboxylate (DMAD) gives quinoline derivative 32. In a similar manner, propargyl amide 29b is converted into ynamide 30b, RCM of which gives bicyclic compounds 31b and 31b in a ratio of 1 to 1 (Scheme 10). 

The palladium catalysed reductive insertion of acetylenes is also viable through the use of formate ions as hydride equivalent (c.f 3.24.), The ring closure of /V-acetylenic-2-iodoanilines gave the corresponding 3-alkylideneindolines in a selective manner (3.27.), demonstrating that the... 

The same strategy was successfully employed in the preparation of the natural product cicerfurane. The procedure started from 4-bromoresorcine and the sesamol-derived acetylene derivative in 3.59.75 One-pot acetyletion, cross-coupling and deprotection followed by spontaneous ring closure led to... 

Not only acetylene derivatives do undergo palladium catalysed intarmolecular carbon-nitrogen bond formation with amines. The similar reaction of olefins in a Wacker-type process also leads to ring closure. (0-Aminopentenes bearing a suitable leaving group in the 4-position were converted to pyrroles in a cyclization-isomerisation-elimination sequence (3.65.),82... 

The low reactivity of ethylene can be overcome by using phenyl vinyl sulfone Ph-S02CH=CH2 instead.853 The PhS02 group can be easily removed with Na-Hg after the ring-closure reaction. Similarly, phenyl vinyl sulfoxide PhSOCH=CH2 can be used as a synthon for acetylene.854 In this case PhSOH is lost from the sulfoxide product (7-12). 

The foregoing mechanism for addition of methylene to the double bond is substantiated by the observation of Jacox and Milligan69 that allene but not cyclopropene is produced upon reaction of CH2 with acetylene in an argon matrix at 4°K. It is quite reasonable to expect in this case that the rate of ring closure would be slow compared to the rate of hydrogen migration. 

The synthesis of 2-arylbenzofurans 226 by heterocyclic ring closure of acetylenic compounds 225 with pyridine hydrochloride is a variation487 of the general method of heterocyclic ring closure of acetylenic compounds 221. 

Diaroylcyclohexadienes which can be prepared from acyclic dienes and dibenzoyl-acetylene undergo ring closure with p-toluenesulfonic acid in benzene to afford benzo[c Jfurans. 

The rearrangement reactions of perfluoroalkadiencs, -cycloalkadienes, -allenes. and -acetylenes have been extensively studied under Lewis acid, Lewis base, pyrolytic and photolytic conditions. In some cases synthetically useful reactions have emerged in most cases, however, mixtures of positional isomers are obtained. Rearrangements resulting in ring closure arc covered in Sections 5.3.2.-5.3.4. 

Other olefinic substrates known to dimerize through photo-induced electron transfer sensitization include enamines (72), diarylethylenes (73-75), vinyl ethers (76), styrenes (77,78), and phenyl acetylenes (79). Alternate ring closures (besides cyclobutanes) are sometimes observed, probably via 1,4-radical cationic intermediates. For example, a tetrahydronaphthalene is formed from the radical cation of 1,1-diphenylethylene, eq. 

Keywords tliioureideo-acetamide, acetylene dicarboxylate, cascade reaction, ring closure, heterocycle... 

A new route to ( )-trachelanthamidine (6) and ( )-isoretronecanol (5) has been reported by Nossin and Speckamp.12 Cyclization of the ethoxy-lactam (28), which is formed on reduction of the imide (27), led to ring-closure with the acetylene to give, after hydrolysis, a 4 1 mixture of the epimers (29) and (30) (Scheme 7). The preferential formation of the five-membered ring is considered to be due to stabilization of an exocyclic vinyl cationic intermediate by the phenylthio group. Reduction of the separated diastereoisomers (29) and (30) yielded ( )-trachelanthamidine (6) and ( )-isoretronecanol (5), respectively. 

An interesting by-product is obtained using a high concentration of acetylene in ethers or esters as solvent an t-lactone (XXX) is formed, probably by insertion into (XXIII) of a second molecule of acetylene, followed by CO insertion and ring closure (52). 

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